In the papermaking process, a paper web is dried by first having excess water pressed from the web as it is transferred on a support felt through nips formed between press rolls in a press section. The paper web is then threaded from the press section to a dryer section where the web is dried as it passes over a series of heated dryer rolls. The web is backed by a porous felt or dryer fabric as it passes over the dryer rolls. In one conventional approach, a single vacuum guide roll is placed between successive dryer rolls in a tier. The felt-supported paper web travels in a serpentine fashion over the first dryer roll, then over the guide roll, then over a second dryer roll to a second guide roll, and so on through the dryer section. The dryer fabric is positioned between the vacuum roll and the web as it moves from one dryer roll to the next, and the reduced pressure drawn on the dryer fabric by the vacuum roll holds the web to the dryer fabric. Due to high web speeds, which in certain applications reach rates of up to 6,000 feet per minute or more, the web may flutter on the felt as it travels between a dryer roll and a vacuum guide roll. This flutter, which can be attributed to disruptive localized pressure generated along the traveling web, detrimentally affects the quality of the paper web produced and can result in web breaks. Breakage of the web being formed results in undesirable machine shut down and lost efficiency while the papermaking machine is being threaded and restarted.
In response to increased web speeds, attempts have been made to both prevent the web from fluttering on the support felt, and to efficiently maximize the drying of the web at the dryer section. Creating a vacuum in a "pocket" formed between two adjacent dryer rolls and the guide roll between them has been helpful in holding the paper web against the felt as it travels between dryer rolls and guide rolls. For example, a vacuum is formed within the pocket by creating a sealing ledge positioned above the guide roll and between two dryer rolls, so that air flow induced on the first dryer roll by the rapidly moving web is deflected to pass up and away from the pocket. Since the felt is permeable, the lower pressure inside the pocket pulls the web against the support felt, thereby preventing flutter.
By using two grooved guide rolls between adjacent dryer rolls, as in U.S. Pat. No. 5,495,679, the disclosure of which is incorporated by reference herein, dryer efficiency can be improved. Two rolls within the pocket allow a greater fraction of each dryer roll to be wrapped by the web, and hence allows greater drying to take place on each dryer roll. The two pocket rolls may be provided with circumferential grooves and enclosed within a box. Drawing a vacuum on the box not only holds the dryer fabric and the web to the grooved rolls, but also retains the web on the dryer fabric as it extends between rolls. The air drawn out of the pocket by the vacuum box or vacuum roll must be replaced. Typically the required make-up air is drawn in exclusively at the front and back sides of the papermaking machine. A papermaking machine can be 200 to 400 inches wide, and thus a rapid inflow of make-up air from the sides can generate turbulence which disrupts the web edges and compromises runnability and paper quality.
What is needed is an apparatus for ventilating the pocket in a paper machine drying section while providing support for the moving web as it travels between dryer rolls.